Image charge relaxation in electrophoretic displays

Abstract

An electrostatic imaging chamber providing a real time visual image. An imaging chamber with electrophoretic particles in the electrode gap, with the particles being selectively moved to a transparent electrode as a result of the electrostatic charge image formed by incoming radiation. An imaging chamber which can be cyclicly operated at a relatively high repetition rate, typically 10 to 20 images per second, thereby providing real time viewing of the object. A conductivity control layer at the gap for discharging the electrostatic charge image each cycle after viewing.

We claim: 
     
       1. In an electrostatic imaging chamber for providing a visual image and having first and second electrodes;   means for supporting said electrodes in spaced relation with a gap therebetween, with said first electrode being relatively transparent optically,   a plurality of electrophoretic particles in said gap, and   means for connecting an electric power source across said electrodes for attracting electrons and negative ions toward one electrode and positive ions toward the other depending upon the polarity of the power source and forming an electrostatic charge image,   the improvement comprising a conductivity control layer at one surface of said gap, with said electrostatic charge image formed at said layer,   with said particles being selectively moved toward said first electrode as a function of said electrostatic charge image forming a visual image viewable through said first electrode, and with said electrostatic charge image being discharged through said layer.   
     
     
       2. An imaging chamber as defined in claim 1 wherein said conductivity control layer has a resistivity in the range of about 10 10  to 10 13  ohm centimeters. 
     
     
       3. An imaging chamber as defined in claim 1 wherein said conductivity control layer has a relaxation time in the range of about 1/100 of a second to about 10 seconds. 
     
     
       4. An imaging chamber as defined in claim 1 wherein said conductivity control layer includes a photoconductor material having a relatively low conductivity state and a relatively high conductivity state. 
     
     
       5. An imaging chamber as defined in claim 4 including means for directing radiation onto said conductivity control layer for switching said layer from the low conductivity state to the high conductivity state. 
     
     
       6. An imaging chamber as defined in claim 5 wherein said layer has a resistivity greater than about 10 11  to 10 14  ohm centimeters when in said low conductivity state, selected according to the desired repetition rate, and a resistivity less than about one-tenth the low conductivity state resistivity when in said high conductivity state. 
     
     
       7. An imaging chamber as defined in claim 6 wherein the resistivity of said layer when in said high conductivity state is less than about one-hundreth the low conductivity state resistivity. 
     
     
       8. An imaging chamber as defined in claim 4 including means for directing onto said first electrode radiation in a first wavelength band to which said photoconductor material is substantially insensitive, with the deposited particles reflecting such radiation, and means for directing onto said first electrode radiation in a second wavelength band to which said photoconductor material is sensitive, with said photoconductor material switching from the low conductivity state to the high conductivity state.   
     
     
       9. An imaging chamber as defined in claim 8 including means for selectively energizing said first and second means. 
     
     
       10. An imaging chamber as defined in claim 4 wherein said first electrode includes a support plate with an electrical conducting layer thereon, and including first means for directing into said plate from an edge, radiation in a first wavelength band to which said photoconductor material is substantially insensitive, with the deposited particles scattering such radiation, and second means for directing onto said first electrode radiation in a second wavelength band to which said photoconductor material is sensitive, with said photoconductor material switching from the low conductivity state to the high conductivity state.   
     
     
       11. An imaging chamber as defined in claim 10 with said radiation from said first means directed into said plate at an angle to produce substantially total reflection of the radiation internally of the plate except for that scattered by the deposited particles. 
     
     
       12. An imaging chamber as defined in claim 4 including means for cyclicly actuating said imaging chamber to provide real time visual imaging and including means for energizing an X-ray source for a short portion of each cycle and simultaneously energizing an electric power source for attracting electrons and positive ions, energizing a first source of radiation in a first wavelength band to which said photoconductor material is substantially insensitive for viewing the deposited particles for a subsequent portion of the cycle, and   energizing a second source of radiation in a second wavelength band for switching said photoconductor material from the low conductivity state to the high conductivity state for discharging said conductivity control layer subsequent to the viewing.   
     
     
       13. An imaging chamber as defined in claim 12 wherein said control means includes means for connecting a relatively high voltage supply to said electrodes while the X-ray source is energized and then connecting a relatively low voltage supply to said electrodes. 
     
     
       14. An imaging chamber as defined in claim 13 wherein said control means includes means for connecting a voltage supply of reverse polarity prior to energizing the X-ray source. 
     
     
       15. An imaging chamber as defined in claim 12 wherein said control means includes means for connecting a voltage supply of reverse polarity prior to energizing the X-ray source.